1. Field of the Invention
The present invention relates to a method for encoding that requires lesser number of bits to express various digital signals (examples are audio signals such as voice, music and image signals), a method for decoding the coded data and apparatus therefor and a recorded medium containing the application programs therefor.
This invention is based on a Japanese Patent Application, First Publication, Hei 11-141653, the content of which is included herein by reference.
2. Description of the Related Art
Methods for compressing information such as voice and image data include a lossy irreversible compression encoding and lossless reversible compression encoding. Known methods for irreversible compression include various methods based on the standards by International Telecommunications Union-Telecommunication standardization sector (ITU-T) and ISO/IEC MPEG. Such methods are capable of compressing the data to less than {fraction (1/10)} of an original size while keeping the loss to a minimum. However, the loss is affected by encoding conditions and input signals, and, depending on the purpose of the work, degradation of the reproduced signal may present a problem. On the other hand, a universal compression encoding often used for compressing computer files and texts is known as a method of reversible compression encoding that can reproduce the original signals perfectly. This method is based on carrying out compression encoding while referring to the statistics of input sequence, and is applicable to any type of signals, and is able to reduce the file size to about xc2xd of the original text, but when applied directly to voice and image data, the extent of compression efficiency is limited to about 20 percent.
Accordingly, irreversible encoding and reversible encoding have conventionally been used separately depending on the type and purpose of the information to be compression. Therefore, it has been necessary to prepare two types of compression files for one piece of information so that the overall compression efficiency cannot be raised and the known methods have been cumbersome to use.
It is an object of the present invention to provide an encoding method for lossless compression of digital signals at high compression efficiency, and a decoding method for decoding the signals compressed by the encoding method to reconstruct the original digital signals.
In the present invention, input digital signals are first quantized with a small number of bits by using lossy encoding. Then an error signal (a signal having a reduced magnitude) between the quantized signal and the original digital signal is compressed by using losssless encoding. The feature that distinguishes the present invention from the conventional methods is that by performing such processes, the method ultimately enables to achieve a higher compression ratio without creating any loss, compared with the conventional methods based on applying reversible encoding and irreversible encoding individually to the data to be compressed.
The present invention enables not only to reconstruct the original digital signals perfectly but also to attain a high compression ratio at the same time. And, because the original digital signal is divided in units of frames by using such synchronizing word, the signal can be reproduced perfectly at anywhere in a compressed encoded sequence. It has been a practice in the industry to record both original codes and compressed codes on one CD-ROM, so that the compressed codes can be offered free of charge but the original codes are made available only with payment. In the present invention, both compressed codes and error codes are recorded so that the compressed codes can be offered free of charge to potential customers while paying customers can reproduce the original sound by having both the compressed codes and the error codes. Because the total data volume of the compressed codes and error codes is significantly less than the total data volume of the original sound and compressed codes, it is possible to record the entire data on a recording medium of a smaller capacity. Although the volumes of irreversible and reverse quantization processes are high, such processes can be performed using highspeed commercial hardwares (MPEG encoding/decoding processors, for example) and software that are readily available in the marketplace at low cost. Therefore, the high volume of process is not a serious impediment to applying the present invention.
Accordingly, the present invention, in combination with music compression encoding techniques such as Twin VQ, MP3, MPEG-4, enables effective utilization of both high efficiency irreversible encoding and lossless reversible encoding at the same time. Therefore, the present invention is particularly attractive to music data storage and delivery purposes, and is applicable in a wide range of applications such as network distribution of music data and recorded media for portable audio devices.